test: increase coverage for stream's duplex

Make use of Arrow function.
Add a small test and this file's coverage is 100%.
https://github.com/nodejs/node/blob/a647d82f83ad5ddad5db7be2cc24c3d686121792/lib/_stream_duplex.js#L25
Coverage: https://coverage.nodejs.org/coverage-067be658f966dafe/root/_stream_duplex.js.html

PR-URL: https://github.com/nodejs/node/pull/10963


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>